If the heater core is leaking, it can allow air to get into the system. If that air pools under the thermostat, (same as if a cylinder head gasket is leaking), it will prevent the thermostat from opening. Thermostats must be hit with hot liquid to open.
When the engine is hot, feel across the radiator to see if there are cold areas indicating some of the tubes are plugged. If the entire surface is hot all the way down to the bottom, the radiator isn't giving up its heat to the surrounding air. That is due to corroded and crumbling cooling fins. Also try pinching the heater hoses. The heater core poses some restriction to coolant flow. With it bypassed, coolant may be circulating too easily through those hoses and is not going through the radiator.
Wednesday, February 19th, 2014 AT 2:15 AM